Conjuction "that"

In fact, “that” in your example is a shortened form of “so that.” You are correct that “so that” is a subordinating conjunction. It always begins a subordinate clause that shows purpose or gives an explanation. It explains how an action produces a result and is used for cause and effect sentences. ”Because” is also a subordinating conjunction, but it would not work in the above sentence. Subordinate clauses, introduced by subordinating conjunctions, cannot stand alone. Let’s look at how your example breaks down.

Clause A (cause): “The movie was high enough on my list.”

Clause B (effect): “I went to see the movie.”

Use “so that” (or “that”) to connect the two clauses in order. 

“The movie was high enough on my list so that I went to see it.”

In order to use “because,” you have to change the order of the independent and subordinate clause. ”Because the movie was high enough on my list, I went to see it,” or, “I went to see the movie because it was high enough on my list.”

In summary, “so that” works with the “effect-clause,” and “because” works with the “cause-clause.”

Best,

Amy